Control device having means for electrically simulating and compensating the inertia momentum of the moving parts of an electrical positioning means

ABSTRACT

The drive has a switching logic which includes a means for producing a correction signal in response to the switching-in of the motor. This correction signal serves to compensate for the kinetic energy of the moving parts of the drive by moving up the lower limit valve of an error signal (derived from the difference between the actual value of the adjusting element position and a fixed value) so as to permit more accurate switching-out and stopping of the motor. Other features are added to prevent overloading, premature switching-out of the motor during starting, short circuits and the like.

United States Patent Inventor Ferdinand Konig Seuzach, Switzerland Appl.No. 6,530

Filed Jan. 28, 1970 Patented Oct. 19, 1971 Assignee Sulzer Brothers,Ltd.

Winterthur, Switzerland Priority Feb. 6, 1969 Switzerland 186/69 CONTROLDEVICE HAVING MEANS FOR ELECTRICALLY SIMULATING AND COMPENSATING THEINERTIA MOMENTUM OF THE MOVING PARTS OF AN ELECTRICAL212,61l,6l2,615,635,648,6l3

(OM/ 417130 N APPARA TUS l [56] References Cited UNITED STATES PATENTS3,283,230 11/1966 Davies et al.. 318/30 3,366,856 l/l968 Sawano 318/448Primary Examiner-Gris L. Rader Assistant Examiner-K. L. CrossonAttorney-Kenyon & Kenyon Reilly Carr & Chapin ABSTRACT: The drive has aswitching logic which includes a means for producing a correction signalin response to the switching-in of the motor. This correction signalserves to compensate for the kinetic energy of the moving parts of thedrive by moving up the lower limit valve of an error signal (derivedfrom the difference between the actual value of the adjusting elementposition and a fixed value) so as to permit more accurate switching-outand stopping of the motor. Other features are added to preventoverloading, premature switching-out of the motor during starting, shortcircuits and the like.

l s s R NW2?" 49 i I AND I asunvr F I l CONTROL DEVICE HAVING MEANS FORELECTRICALLY SIMULA'IING AND COMPENSATING TIIE INER'IIA MOMENTUM OF THEMOVING PARTS OF AN ELECTRICAL POSITIONING MEANS This invention relatesto an adjusting drive and more particularly, to an adjusting drivehaving an asynchronous motor for driving adjistable elements.

Adjusting drives have been known in which asynchronous motors have beenused to drive various adjusting elements, such as valves, through theintennediary of a gearlike or hydraulic transmission. Such motors havebeen controlled by switching-in and switching-out the rotating field.For example, by using an error signal derived from a comparison of adesired value signal with an actual value signal of the position of theadjusting element, when an upper limit value is exceeded, the motor isswitched in and when a lower limit value is fallen short of, the motoris switched out. The sign of the error signal has been used to determinethe direction of rotation of the motor.

Generally, a number of requirements are simultaneously imposed on anadjusting drive, as above, for an adjusting element such as an organ orvalve regulating the flow in a conduit of a steam-power plant which isdriven by alternating or multiphase current. The most important of therequirements are the greatest possible accuracy of adjustment, for thepurpose of keeping regulatory variations and switching frequency as lowas possible; at the most, a creeping movement of the adjusting elementwithin the range of the adjusting tolerance under the effect ofstatic-adjusting forces, which for example originate from a mediumflowing through the conduit; and, finally, high performance, that is,great torque of the motor with high adjusting speed and rapid braking,without any particular heat-overloading of the motor. The aforesaidrequirements are preferably to be fulfilled in such a way that a minimumof connecting conductors are required between the control contrivanceand the adjusting organ. In particular, signal conductors for thesetting or positioning end switches and mechanical load-switches are tobe avoided.

Accordingly, it is an object of the invention to provide the greatestpossible accuracy for the adjustment of an adjusting drive.

It is another object of the invention to satisfy the remainingrequirements mentioned above with various supplemental provisions.

The invention is characterized in that means are provided which, uponbeing made operative by the switch-in signal for the motor, produce acorrection signal whose value depends on the prevailing duration of theswitch-in of the motor and which reproduces the momentary kinetic energyof the moved parts and changes the lower limit value for the switch-outpoint of time. By means of the correction signal for shifting theswitch-out point of time, and which takes into account an elimination ofthe mechanical inertia of the moved parts, the accuracy of adjustment ofthe adjusting element is substantially improved.

This result is particularly necessary in the frequently occurring cases,with adjusting elements of the aforesaid kind, in which continuousdisplacement of the adjusting element is necessary as the adjustingelement seldom reaches its top speed. Adjusting accuracy at high speedscan be improved if a source of direct voltage with suitable switchingmeans is provided for the braking of the moved parts. This directvoltage is superimposed on the motor stator during the switching-out ofthe rotating field. In addition, switchover means can be providedthrough which the braking direct voltage applied to the motor becomesdiminished after the motor has come to a standstill. The chief result ofthe diminution of the braking voltage is a diminution of the danger of aheat-overload of the motor.

Furthermore, provision has to be made so that the adjusting drive, atthe time of the high starting-up moment of the motor in its endpositions, does not become overstressed mechanically. Therefore, acurrent-measuring device may be provided for the motor current in orderto produce an output signal which, when an output limit value isexceeded, supplies a dominating switchout signal to the switching means,for example, consisting of controlled reetifiers (SCR), for the motorcurrent. This measuring device, on the one hand, serves to prevent aheat-overload on the drive motor, and, on the other hand, simultaneouslyavoids mechanical overloading of the entire adjusting drive.

In order to make it possible to prevent a switchoff of the motor by theaforesaid arrangement during the starting-up phase, supplementary meansmay be provided, which may, for example, consist of a monostablemultivibrator having a certain time constant, by means of which theoutput limit value for a predetermined time interval of the startupphase is raised.

An additional means may also be provided which is influenced by the signand value of the actualvalue signal for the adjusting element to permitan increase of the output limit value beyond the predetermined timeinterval. This additional means thus allows starting during a so-calleddifficult startup, during which the starting-up phase becomes increasedbeyond the predetermined time interval through additional mechanicalforces or resistances, for example, through additional friction andadditional pressure forces, particularly during the lifting of a settingvalve or organ off its seat when starting-up in the opening directionwhile preventing switching-off of the motor.

It is moreover advantageous if the output-dependent switchout signaltriggers an alarm device in the event that the actual-value signal forthe setting of the adjusting element does not indicate a specified endposition for the adjusting element.

In order to improve the starting-up of the motor and to prevent shortcircuits in the motor winding between the braking and stopping voltages,respectively, and the imposed alternating voltage, and to protect thecontrolled rectifier used for example as a switch from overloading, acomparing organ is provided. This comparing organ compares with oneanother the instantaneous values of the switched phases of thealternating voltage to be switched, and triggers a releasing or freeingsignal for the switch-in and switchout of the rotating field only whenthe instantaneous values of these phases are equal and positive.

Finally, the motor may be equipped with a controlled brake and/orself-restraining (i.e. self-locking) mechanical or hydraulictransmission. This makes it possible to relieve the motor in its stoppedstate to a great extent from mechanical forces, and thus serves to lowerthe direct current flowing through the motor so that the heat loading ofthe motor is lowered.

These and other objects and advantages of the invention will become moreapparent from the following detailed description and appended claimstaken in conjunction with the accompanying drawings in which:

FIG. 1 schematically illustrates an adjusting drive with a valve forregulating the flow-through, the associated motor, andthe switchinglogic for controlling the motor according to the invention;

FIG. 2 graphically illustrates the manner in which the arrangement ofFIG. I functions, as a function of time;

FIG. 3a schematically illustrates the electric circuitry of a comparingorgan utilized in the arrangement of Fig. l; and

FIG. 3b graphically illustrates the manner of operation of the comparingorgan.

Referring to Fig. l, the adjusting drive 1 comprises an asynchronousmotor 2, which through the intermediary of a screw-drive 3 adjusts avalve spindle 4 carrying a valve body 5 of a valve in a flow-throughconduit 6. The valve spindle 4 is connected over a suitable means suchas a lever with a mechanical-electrical transducer 7 such as anactual-value transmitter in order to translate the movement of thespindle 4 to the transducer 7. The transducer 7, in turn, acts over asignals conductor 8 on a switching logic 10 to deliver an actual-valuesignal 1' thereto corresponding to the position of the spindle 4 andthus the flow. Electric voltages are used throughout as signals in theswitching logic 10.

The asynchronous motor 2 is connected by three conductors l l, 12 and13. Over a current-measuring device 15, directly/or by switching means14, to a three-phase network 16. A main switch (not shown) may also beinstalled between the device 15 and the connected network 16.

The conductors l 1, 12 are also connected directly by means of aconnecting conductor 21, or by means of a conductor 22 and switchingmeans 23, 24, with a source 25 of direct voltage, which is supplied overthe conductor 26, 27 from the threephase network 16.

The direct voltage source 25 consists, for example, of a transformer 28,at whose output direct voltages as indicated schematically, are producedthrough rectifiers 29. Compared with the output terminals 210 for theconductor 21, the output 320 which is connected with the conductor 22over the conductor 32 and the switch 23 has, for example, a voltage of+40 volts. This serves for braking of the motor 2 at the switchout ofthe rotating field.

The voltage at output 33a which is also connected with the conductor 22over the conductor 33 and the switch 24 is lower in comparison with thatof the output 32a, and amounts, for example, as compared with point 21a,to about volts. This second voltage is the lowered braking or stoppingvoltage for the motor 2 during standstill. As previously mentioned, theswitches 23 and 24, and also the switches 43, 57 and 58, which aredescribed below may be made as controlled rectifiers (SCR) which becomeignited by pulses emitted by the switching logic l0.

The current-measuring device 15 has for each phase R, S and T of thethree-phase current, a measuring transformer 34, the secondary windingof each of which is completed by a resistance 35, and has a connectionto the zero or reference potential 82 of the switching logic 10. Throughthe intermediary of diodes 36, the potential which is dependent on thecurrents flowing through the resistances 35 is conducted at the points37 in parallel to the output 38 of the measuring device 15. In this way,the instantaneous value of the voltage U38 appearing at the output point380 is equal to the maximum value of the currents flowing at thatinstant in the conductors ll, 12

and 13. There will be described later the further utilization and theoperation of the voltage U38 forming the output signal p of themeasuring device 15, and there will also be explained the purpose andthe operation of the voltage-divider formed by the resistances 41 and42, and also of a switch 43.

A comparing organ 30 is also connected to the three-phase network 16 inparallel with the motor 2 by way of conductors 1 la, 12a and 13a. Thiscomparing organ 30 produces a signal m, in the form of a briefvoltage-pulse, which is introduced into the switching logic 10 through aconductor 31. This signal m serves as a freeing signal for the switch-inand switchout of the rotating field, and is produced when, and onlywhen, the instantaneous values of the voltages of both the phases S andT which are to be switched are equal and positive. The construction andoperation of the comparing organ 30 are described below in connectionwith Figs. 3a and 3b.

The switching logic 10 wherein the signals flow-diagram is shown by theusual symbolic representation and which is mainly made with elementsknown in digital technology, has a desired-value signal s suppliedthereto through a signal-conductor 40. This signals together with theactual-value signal i act on an apparatus 44 for comparing the desiredand actual vale signals. In this comparison apparatus 44, an errorsignal a is formed from the difference between the two input magnitudess and i, and flows to the branch point 45. From this point 45, the errorsignal a goes to a proportional amplifier 47 of known design, whichforms the absolute value b of the error signal. The absolute valuesignal b then flows to a limit-value emitter 46 having a hysteresiselement of known design.

The limit-value emitter 46 in the present control circuitry firstperforms a conversion of the signals a and b, previously formed asanalog signals at the input, into a digital output c and then operatesin such a way that in the case of a rise of the input signal whileexceeding a first higher limit value G1, the output signal 0 jumps fromO to L. The output signal c, however, jumps back from L to 0 only when asecond lower limit value G2 is fallen short of. The pattern of theoutput signal 0, as a function of the algebraic sum oft/1e input signalsb, d and e, therefore has the form shown in Fig. l of a hysteresis loop,from which is derived the designation of this known switching element46.

The other input signals supplied to the limit-value emitter 46 are apredetermined negative limit-value signal e, which influences themagnitude of the limit value G1 needed for the appearance of an outputsignal c, and a positive correction signal d which depends on theprevailing switch-in duration of the motor 2. This signal d takes intoaccount the kinetic energy of the moved parts of the adjusting drive.The signal d is triggered by the output signal 0 of the signal-emitter46, which signal in turn becomes branched at a point 48, and on the onehand flows to an AND element 49, and on the other hand flows by way of aNOT or inversion element 50 of known design as a signal E (not 0 "or "cinverse") to an RC element 51 acting as an integrator. The signal Ffromthe inversion element 50 also arrives at another AND element 52.

The RC element 51 acts in the present control circuitry so that at theoccurrence of a E signal at its input, the signal d becomes integratedas a chronological integral of this appearance of 5 up to its limitvalue, which is determined by the magnitude of the resistance R and ofthe capacity 0 thereof.

Through this, the signal d keeps its positive maximum value so long asthere is no output signal c from the limit-value emitter 46, which formsone of the switch-in requirements for the rotating field, as will bedescribed farther on.

At the appearance of the signal 0, the input signal 6 at the input ofthe RC element 51 vanishes, and this element's condenser begins, inaccordance with the familiar pattern of condenser discharging, to becomedischarged. The sum of the two positive inputs b and d of thelimit-value emitter 46 thereby becomes smaller, dependent on theprevailing switch-in duration for the motor 2, depending, that is, onthe duration of the signal c. Because the signal b has becomeretrogressive in magnitude, through the switching-in of the motor 2, theinput signal of the limit-value emitter 46, as the sum of the signals b,d and e, become smaller, until upon reaching the limit value 02 themotor, through the vanishing of the switch-in requirement c, becomesswitched out. G2 hereby becomes reached the more rapidly the farther thecondenser of the RC element has become discharged. Through this,however, the limit value G2, and thus the switchout point of time forthe motor 2, becomes shifted to the right in the graph dependent on andwith the increasing duration of switch-in, relative to its value atmaximum signal d. This measure increases the accuracy of adjustmentsubstantially for the adjusting drive 1, because the kinetic energyexisting in the moved parts is taken into account during the braking ofthe motor 2.

It should furthennore be mentioned that a conversion takes place in theRC element 51 of the digital input signal E into an analog signal d,which is made regressive in the limit-value emitter 46. The function d=f (t) shown in the symbol of the RC element 51, is, as is the generalcustom, represented for a pattern of the output signal with a unit-jumpof the input signal Eat the zero point.

The two AND elements 49 and 52 act on a storage device 53, for example abistable circuit flip-flop of known design. This storage 53, with twoinputs A and B and two outputs X and Y, has a definite switch-inarrangement; that is, with a switchon of its supply voltage, the device53 is set into the basic position (as shown) in which an output signal happears at the output X.

In order to set the storage device 53 into its other position, it isnecessary that a signal appear at input B. Such a signal appears as soonas all four switching requirements appear at the input of the ANDelement 49. Together with the signal 0, these requirements are a signalFwhich indicates that the motor 2 is at a standstill, a signal m thatindicates that in the comparing organ 30 the condition U =U exists, anda signal n which appears as an output signal from a second storagedevice 70 of similar design when the device 70 is reset through itsinput A into that position which gives a signal at its output X.

Aside from the switchon of its supply voltage, the storage device 53 isreset into the basic position through an output signal from the ANDelement 52 applied to its input A, whereby for the appearance of thisinput signal conditions F and m have to be fulfilled.

The signal 1 at the output Y from the storage device 53 arri'ves at twoAND elements 54 and 55, whose output signals act, through theintermediary of ignition devices 56, 59 on rectifiers 57, 58 in theswitching means 14 which serve as switches, in the conductors l2, l3 andthus effect a switch-in of the rotating field either in the closeddirection (AND element 54, ignition device 59 and switch 57), or in theopened direction (AND element 55, ignition device 56 and switch 58) ofthe valve 5.

The ignition of one or the other of the two switches 57, 58 depends onthe sign of the error signal a. A trigger 60, at whose input thereappears the analog error signal a coming from the branch point 45, formsa signal r reproducing the sign of the signal a. The output signal R atits digital output appears only when the sign of the error signal a ispositive; that is, when the desired value s at the input of thecomparison device 44 is greater than the actual value i. The sign signalr arrives, as a second switching condition at the AND element 55 for theswitch-in of the motor 2 in the opened direction of the valve 5.Furthermore, the signal r is, through the intennediary of a NOT element61, conducted as signal Fto the AND element 54 as a second input signalfor the switch-in of the motor 2 in the opposite direction. Finally, thesignal r is also fed to another AND element 62, whose function will bedescribed below.

A third switching condition is also imposed on the AND element 55 forthe ignition of the switch 58. This third signal u is formed in atrigger 63 as a digital output signal so long as there has not appearedat its input an actual value signal i of a fixed desired value thatcorresponds to a I00 percent opening of the valve 5. The function of thesignal u is to prevent a startup of the motor 2 in the opening directionin the event that the valve 5 is already fully open. The signal ufurthermore arrives at an indicating instrument (not shown) which at thevanishing of this signal u and indicates the valve position as being 100percent open."

As has already been described, the storage device 53 with the presenceof a signal at its input A is reset into a definite switchon or basicposition. Through this, however, the signal 1 at the output Y vanishesthus causing either switch 57 or 58 to be opened; that is, thecontrolled rectifier 57, 58 becomes extinguished because of thealternating voltage applied thereto.

The signal h, which appears at the output X of the storage device 53 inthe basic position, serves to switch in the braking voltage for themotor 2. This signal h arrives, by way of a timing element 64, as asignal It at an ignition device 65 for the controlled rectifier orswitch 23, through which the braking voltage U23 of about 40 voltsbecomes switched to the motor 2. The timing element 64, constructed as amonostable multivibrator of known design, at the appearance of an inputsignal h produces an output signal k, which either in the presence of aninput signal h vanishes after a certain time or else vanishessimultaneously with the vanishing of the signal h. For this reason, thepatterns of the input signal h and of the output signal k of the timingelement 64 are plotted in Fig. l as a function of time. The timeconstant; that is the duration of the output-signal pulse k, is herebyestablished so that a signal It exists until the motor 2 has been brakedfrom its maximum attainable speed to a standstill. This duration of thesignal pulse k is always sufiicient to brake the motor 2 when the motorhas only reached some lower speed.

The signal k appears simultaneously at a NOT element 66, so as then toarrive as a signal k at the AND element 67. The signal k also istransmitted to the AND element 49 to form a condition for the setting ofthe storage device 53 out of the basic position and thus to fonn theswitch-in condition for the motor 2, to brake the motor 2 to a stopbefore starting up again. At the AND element 67, the signal 75, togetherwith the signal h, fonns the switchon condition for the switch 24, bymeans of which, through the intermediary of the ignition device 68, thediminished direct or holding voltage U24 becomes switched to the motor 2after the motor has come to a standstill.

The relatively short time during which the entire braking voltage U23 isapplied to the motor 2, and the following transition to the lowerholding voltage U24, protect the motor 2 against heat-overloads during astandstill of the adjusting element 5. Upon the vanishing of the signalh, at the instant of the switch-in anew of the rotating field, theswitch 24, through the alternating voltage of the rotating fieldreaching it, becomes automatically extinguished, so that the holdingvoltage at that instant becomes switched ofi from the motor 2.

As has been mentioned, the current-measuring device 15 serves to protectthe motor 2 against excessive current. To this end, the potential U38appears at the point 38a as an output signal p from this device. Thissignal p is fed, as an analog input signal, to a trigger 69, at whosedigital output a signal w appears as soon as the positive signal pexceeds a second negative input signal q which represents the maximumallowable current I, for the motor 2. The output signal w of the trigger69 arrives at the input B of the second storage device 70 which isconstructed similarly to the storage device 53.

By means of the signal w at the input B; the storage device 70 is setout of the illustrated switchon or basic position into a second stableposition, in which an output signal y appears at an output Y. Thissignal y is fed to two AND elements 71 and 72.

The setting of the storage device 70 brings about a vanishing of thesignal I: at the storage output X, and thus a vanishing of one of thefour switch-in conditions or requirements at the AND element 49, whichinitiates a tilting of the storage device 53, and thus a switching-in ofthe rotating field. ln this way, that is through the vanishing of thesignal n, a dominating switchout signal is give for the rotating fieldby the currentmeasuring device 15 as soon as the device 15 measures anexcessive current for the motor 2.

As is well known, during starting-up phase a motor needs a greatercurrent. This is taken into account in that, by transmitting theswitch-in signal 1 to a timing element 73, designed similarly to thetiming element 64 but having a shorter duration of pulse for its outputwith the existence of an input signal 1, the switch 43 is actuatedthrough the intermediary of an ignition device 74. Through this, for theduration of the pulse of the output signal x of the timing element 74,the signal p at the input of the trigger 69 becomes divided through thevoltage dividers 41 and 42 in proportion to these resistances, so thatfor this length of time the input signal p of the trigger 69 isartificially diminished. This ensures that during the starting-up phasea current which exceeds the desired value I, can flow through the motor2.

in the particular case of a closed valve 5, the prescribed time intervalof the starting-up phase may not suffice to move the valve in thedesired direction during the starting-up in the opening direction whenthere is a so-called difficult start." Therefore, for this case, thestartup phase can be lengthened so as to prevent a switching-out of therotating field by the current measuring device 15. To this end, atrigger 75 with a digital output is actuated from the actualvalue signalsignal i and a fixed desired-value signal 2 percent." The output signalv of the trigger 75 appears as soon as the actual-value signal 1'reaches the fixed desired value 2 percent." The term "2 percent" hasbeen selected as an allowable tolerance range for the indication valveclosed," because it is clifficult to determine the exact value 0percent." Within the range from 0 to 2 percent, the valve is thereforeregarded as being closed, so long as the storage device 70 gives asignal y at its output Y.

The signal v arrives at the AND element 62 and together with the signalr reporting the sign for starting up in the opening direction, makespossible the appearance of a signal 2 at the output, which signal istransmitted to an OR element 77. By means of this OR element 77, havinganother input to which a reset signal is supplied by a manual key (notshown) an input signal is supplied to the input A of the storage device70. This input signal serves to hold the storage device 70 in its basicposition or serves to reset the storage device 70 into that position.Because of the special design of the storage device 70, an input signalat the input A in any event provides an output signal It at the outputX, independently of whether a signal w is or is not present at the sametime. This, however, in the first place allows by means of the manualkey (not shown) the control arrangement, and thus the motor 2, to becomeoperative again, for example, after a derangement. In the second place,a switching-out during difficult starting is prevented during thepresence of a signal z This means that so long as the valve is in itsclosed region of2 percent, and a sign signal r is given for starting-upin the opening direction, a signal n is present, and in spite of excesscurrent, the motor 2 does not become switched out.

The signal v moreover arrives at the AND element 71, whose outputsignal, in the case of the simultaneous presence of the excess-currentsignal y at the output Y of the storage device 70, arrives at anindicating device (not shown) for indicating valve closed. The signal vis also passed to the inversion element 76 and becomes inverted fortransmission as the signal V at the AND element 72. If, at the input tothe AND element 72 there are simultaneously present the signals v and y,that is, ifexcess current is indicated for the motor 2, without thevalve 5 being in its2 percent closed range, then the out put signal ofthis AND element 72 operates an alarm device (not shown) of an opticalor acoustic nature, for the purpose ofindicating that a derangementexists in the adjusting drive.

The adjusting drive as shown functions, for example, as is showngraphically in Fig. 2. That is, over a time axis, up to the instant tthe adjusting drive 1 is at rest so that the desired value x deviates,for example, by a small amount from the actual value i. From the instantI, onward, the desired value 5 ascends on a slope to the instant 1,,after which the signal s is again fixed. The error signal a or b, whosezero line is designated 82, ascends correspondingly from the instant tuntil reaching the limit value 01 at the instant 1,. The limitvalueemitter 46 at that instant emits an output signal 0, which, in the eventthat signals Fand n are present, at the instant of the appearance of afreeing signal m, coming from the comparing organ 30, by the aid of theAND element 49 sets the storage device 53, and thus releases a signal 1.

When there is fulfillment of the other switching conditions (sign signalr and signal 14), the output signal of the AND element 55 by means ofthe ignition device 56 ignites the switch 58. The asynchronous motor 2then runs in the opening direction. After a short time, the motor 2reaches the full speed of stroke corresponding to the straight line 81at With the appearance of the signal c, the RC element 51 begins todischarge in the described manner, and shifts the limit value G2 upward,and in this case namely as far as its upper end-value, because the motor2 reaches its maximum speed.

The difference between the actual value 1' and the desired value 3becomes diminished because of the travel of the valve body 5 to itsdesired value, until the difference falls short of the limit value G2.At this instant, t the signal 0 vanishes; and at the appearance of thenext signal m the storage device 53 is reset to its basic position; theswitch 58 becomes switched out, and at the same time, the direct voltageswitching means 23, through the newly appeared output signal h, of thestorage device 53 is switched on for a length of time dependent on thetiming element 64. After the expiration of this time, the switchingmeans 23 opens, while the direct-voltage switching means 24 closesbecause of the presence of the signals? and it.

Because of the appearance of a direct-voltage at two terminals of theasynchronous motor 2, the motor 2 becomes braked hard until stopping atthe instant Following this, the motor 2 is held against running by meansof the diminished direct voltage supplied through the switching means 24while the heat-loss output of the motor 2 is carried off throughdissipation without any unallowable rise in temperature.

Because the desired value s is still increasing, the magnitude of a,i.e. signal b again increases, until, at the instant I.,, thethree-phase switch 58 becomes switched in a second time in the describedmanner. At the instant which has in Fig. 2 been chosen in such a waythat the motor at that time has still not reached the top speed ofstroke, the three-phase switch 58 again becomes switched off, becausethe error signal b at this instant (not having reached its maximumvalue) falls short of the limit value G2. At the instant t the motor 2,as has already been described, is braked in two stages with directvoltage and held still so that at time a small regulatory deviation isonce more obtained between the actual value 1' and the desired value s.

Below the graphs of the signals i and s and of the signal b,respectively there are plotted in Fig. 2 the switching positions of theswitches 58, 43, 24 and 23, for the various instants of time t to thathave been described. The switchon conditions of the said switches arehereby designated by the symbol L, and their switchoff states by thesymbol 0.

Fig. 3a shows the electrical circuitry for the comparing organ 30,through which the pulselike signal m is given to the two AND elements49, 52 at the instant of the transmission of the corresponding switch-inand switchout signals for the rotating field. The requirement for theappearance of a signal m is that the two-phase voltages U and U beequal; that is, their potential difference U from one to the other isequal to zero. In addition, U and U should be positive. Thus, negativevalues U ,=U 0 are not used for releasing a pulse m.

The function of the comparing organ 30 is to make possible a switch-inor switchout of the rotating field only at instants that result from theaforesaid coincidence of U and U and the supplementary conditionpositive," for the purpose of preventing short-circuits due to defectiveignition and overloadings of the rectifiers which are used andcontrolled as switches.

The two phases S and T are fed through the conductors 13a, 120 (Fig. l)to the primary winding ofa transformer 85, while the phase R isconnected, through a conductor 11a and the primary side of anothertransformer 86, with the phase S. The secondary side of the transformeris completed by two diodes 87, 88, so that at a point 89 the absolutevalue EU I of the potential difference U appears, having only positivehalfwaves. This absolute value serves as the control voltage at the baseof an NPN transistor 90. This transistor 90, has, through a resistance91, its collector at a positive potential relatively to its emitter. Theemitter in its turn is connected with a middle tap 83 of the secondarywinding of the transformer 85. The middle tap 83 in turn is at thereference potential 82 of the complete arrangement. A pair ofresistances 92, 93 form a voltage divider for the voltage U Thesecondary winding of the transfon'ner 86 is connected in parallel, byway of a diode 94 and a resistance 95, to the base-emitter section ofthe transistor 90. At its output there appears the alternating voltage Uof which, because of the diode 94, only the positive half-waves arriveat the base of the transistor 90. The desired signal m appears at theoutput 96 of the switching arrangement and flows to the two AND elements94 and 52,

The way in which the comparing organ 30 operates as shown in Fig. 3a, isas follows: so long as the potential of the transistor base is positive,this transistor is conductive and its output 96 is at approximately thepotential of the middle tap 83, that is, at the zero potential of thecomplete arrangement. A signal at the output 96 can therefore not appearduring this time.

If the effect of the voltage U is disregarded, then, from what has beensaid in the foregoing, an output signal m becomes produced when nopositive potential is applied to the transistor base, which is the casefor IU I -0, that is when U =U whether positive or negative.

The emission of the signal m for the transmission as the desired signalm occurs only for U =U Q, by the aid of the half-wave U which areapplied as positive voltages, that is, as voltages which move the output96 to zero potential when pulses m occur as a result of IU I for U ,=U0.

Because the negative half-waves of U do not appear due to the diode 94,there are obtained as desired, at output 96 signals m for U =U 0.

Fig. 3b shows, as a function of the time t, the relative phase positionsand the pattern of the voltages U U and U U |U m; U and m. In therepresentation of the three phases R, S and T, solid dots indicate theinstants for the signals m, and small circles indicate the instants forthe sup pressed signals m.

It should also be mentioned that the invention is not limited to thedescribed example. It is, for example, possible to also make theswitching logic as a control in analog technique, or, for example, toreplace the controlled rectifiers by mechanical switches controlled froma solenoid. The asynchronous motor 2 may moreover be driven bysinglephase or two-phase alternating current, instead of by threephasecurrent.

What is claimed is:

l. A positioning drive for moving an element to be positioned having anasynchronous motor with a rotating field for moving said element,switching means for switching in and switching out said rotating fieldof said motor, and means for comparing an actual value signalcorresponding to the position of said element and a desired value signalcorresponding to a predetermined value to produce an error signalcorresponding to the difference between said value signals whereby withthe sign of said error signal determining the direction of rotation ofsaid motor, said rotating field is switched in in response to said errorsignal exceeding an upper limit and switched out in response to saiderror signal falling short of a lower limit characterized in having ameans (51) for producing a correction signal (d) in response to theswitching in of said rotating field of said motor of a valuecorresponding to and simulating the momentary kinetic energy of themoved components and depending on the prevailing duration of theswitch-in of said motor to shift said lower limit for the switching-outpoint of time of said rotating field of said motor in a directiontowards an earlier switchout with an increasing duration of theswitch-in of said motor.

2. A positioning drive as set forth in claim 1 wherein said motor has astator and which further includes a source of direct voltage and aswitch means for switching said source to said stator in response toswitching-out of said rotating field of said motor to brake said movedparts.

3. A positioning drive as set forth in claim 2 further including meansfor diminishing the direct voltage supplied to said stator after saidmotor stops.

4. A positioning drive as set forth in claim 1 further including acurrent-measuring device for measuring current supplied to said motorand producing a performance signal in response thereto and means forproducing a dominating switchout signal in response to said performancesignal exceeding a predetermined limiting value for activating saidswitching means to switchout said rotating field of said motor.

5. A positioning drive as set forth in claim 4 further including meansfor raising the value of said limiting value for a predetermined periodof time during starting-up of said motor.

6. A positioning drive as set forth in claim 5 further including meansresponsive to the sign and magnitude of said actual value signal forallowing the raising of said limiting value beyond said predeterminedperiod of time.

7. A positioning drive as set forth in claim 5 further including analarm device responsive to a switching-out signal of said switchingmeans upon said actual value signal indicating a predetermined endposition of said ad usting element being exceeded.

8. A positioning drive as set forth in claim 1 wherein a means isconnected to said motor and said switching means to supply one of twophases of alternating current to said motor, and which further includesa comparing organ for comparing momentary values of said two phases andemitting a freeing signal to said switching means in response to saidmomentary values being equal and positive.

9. A positioning drive as set forth in claim 1 wherein said motor has acontrolled brake.

10. A positioning drive as set forth in claim 1 further including atransmission between said motor and said adjusting element.

11. A positioning drive as set forth in claim 10 wherein saidtransmission if self-locking.

12. A positioning drive for controlling the operation of an asynchronousmotor driven over a single or multiphase alternating voltage source,said motor being connected to an element to move said element; saidpositioning drive including a switching logic connected between saidvoltage source and said motor to selectively switch-in and switchoutsaid motor to said voltage source,

said switching logic having a means for emitting an error signal inresponse to a difference between an actual value signal corresponding tothe position of said element and a predetermined desired value signal,means responsive to said error signal exceeding an upper limit value forswitching in said motor and to said error signal falling short of alower limit value for switching out said motor, and

means responsive to a switching-in of said motor for producing acorrection signal depending on the prevailing duration of theswitching-in of said motor and representing the momentary kinetic energyof said motor and element to increase said lower limit value towardssaid upper limit value while increasing the duration of the switch-in ofsaid motor.

13. A positioning drive as set forth in claim 12 wherein said lattermeans is an RC element acting as an integrator, wherein the condenserthereof is charged during the switch-out of said motor and dischargedupon switching-in of said motor.

14. A positioning drive as set forth in claim 12 wherein said lattermeans is an integrator, wherein the condenser thereof is charged duringthe switchout of said motor and discharged upon switching-in of saidmotor.

1. A positioning drive for moving an element to be positioned having anasynchronous motor with a rotating field for moving said element,switching means for switching in and switching out said rotating fieldof said motor, and means for comparing an actual value signalcorresponding to the position of said element and a desired value signalcorresponding to a predetermined value to produce an error signalcorresponding to the difference between said value signals whereby withthe sign of said error signal determining the direction of rotation ofsaid motor, said rotating field is switched in in response to said errorsignal exceeding an upper limit and switched out in response to saiderror signal falling short of a lower limit characterized in having ameans (51) for producing a correction signal (d) in response to theswitching in of said rotating field of said motor of a valuecorresponding to and simulating the momentary kinetic energy of themoved components and depending on the prevailing duration of theswitch-in of said motor to shift said lower limit for the switching-outpoint of time of said rotating field of said motor in a directiontowards an earlier switchout with an increasing duration of theswitch-in of said motor.
 2. A positioning drive as set forth in claim 1wherein said motor has a stator and which further includes a source ofdirect voltage and a switch means for switching said source to saidstator in response to switching-out of said rotating field of said motorto brake said moved parts.
 3. A positioning drive as set forth in claim2 further including means for diminishing the direct voltage supplied tosaid stator after said motor stops.
 4. A positioning drive as set forthin claim 1 further including a current-measuring device for measuringcurrent supplied to said motor and producing a performance signal inresponse thereto and means for producing a dominating switchout signalin response to said performance signal exceeding a predeterminedlimiting value for activating said switching means to switchout saidrotating field of said motor.
 5. A positioning drive as set forth inclaim 4 further including means for raising the value of said limitingvalue for a predetermined period of time during starting-up of saidmotor.
 6. A positioning drive as set forth in claim 5 further includingmeans responsive to the sign and magnitude of said actual value signalfor allowing the raising of said limiting value beyond saidpredetermined period of time.
 7. A positioning drive as set forth inclaim 5 further including an alarm device responsive to a switching-outsignal of said switching means upon said actual value signal indicatinga predetermined end position of said adjusting element being exceeded.8. A positioning drive as set forth in claim 1 wherein a means isconnected to said motor and said switching means to supply one of twophases of alternating current to said motor, and which further includesa comparing organ for comparing momentary values of said two phases andemitting a freeing signal to said switching means in response to saidmomentary values being equal and positive.
 9. A positioning drive as setforth in claim 1 wherein said motor has a controlled brake.
 10. Apositioning drive as set forth in claim 1 further including atransmission between said motor and said adjusting element.
 11. Apositioning drive as set forth in claim 10 wherein said transmission ifself-locking.
 12. A positioning drive for controlling the operation ofan asynchronous motor driven over a single or multiphase alternatingvoltage source, said motor being connected to an element to move saidelement; said positioning drive including a switching logic connectedbetween said voltage source and said motor to selectively switch-in andswitchout said motor to said voltage source, said switching logic havinga means for emitting an error signal in response to a difference betweenan actual value signal corresponding to the position of said element anda predetermined desired value signal, means responsive to said errorsignal exceeding an upper limit value for switching in said motor and tosaid error signal falling short of a lower limit value for switching outsaid motor, and means responsive to a switching-in of said motor forproducing a correction signal depending on the prevailing duration ofthe switching-in of said motor and representing the momentary kineticenergy of said motor and element to increase said lower limit valuetowards said upper limit value while increasing the duration of theswitch-in of said motor.
 13. A positioning drive as set forth in claim12 wherein said latter means is an RC element acting as an integrator,wherein the condenser thereof is charged during the switch-out of saidmotor and discharged upon switching-in of said motor.
 14. A positioningdrive as set forth in claim 12 wherein said latter means is anintegrator, wherein the condenser thereof is charged during theswitchout of said motor and discharged upon switching-in of said motor.